But the sudden withdrawal of the Russian world number two earlier on Wednesday leaves only Radwanska, who will play German Mona Barthel in an early match on Thursday, as a threat.

Sharapova, the Olympic silver medallist, withdrew from the August 4-13 Montreal event citing a stomach virus that had been bothering her since the London Games.

For Radwanska to reach the top of the world rankings for the first time, she needs to win the $2 million tournament and hope Azarenka is eliminated before the quarter-finals.

Sharapova, replaced in the draw by Galina Voskoboeva, said she initially felt unwell a day before her 6-0 6-1 loss to Serena Williams in the gold medal decider last Saturday but that the illness had worsened since arriving in Canada.

"I came to Montreal hoping that I would recover but this morning I was still not very well, so I was not going to be able to practice which is, you know, not the best preparation in order to compete tomorrow," she said.

"We want to have the best preparation we can for the US

Open and this is a really big tournament for us. Personally, I was here in order to play the tournament. I was ready but my body is just not ready for it, so you kind of have to make the smart decisions for your body."

Petra Kvitova and Sam Stosur, who was runner-up to Williams in this tournament last year, were both successful in their first matches at the event on Wednesday night.

Stosur overcame a 1-5 deficit and then three set points to claim the first set over Simona Halep 7-5. Then, after drawing level at 4-all in the second set, she progressed when her Romanian rival forfeited with a leg problem.

Kvitova had a greater scare when within a game of losing to Ksenia Pervak.

The 2011 Wimbledon champion trailed by a set and 5-6 before finding her form to defeat Pervak 4-6 7-6 (7-0) 6-0.

Canadian teenager Eugenie Bouchard gave local fans their biggest thrill of the day when she upset Shahar Peer 3-6 6-2 7-5 to join compatriot Aleksandra Wozniak in the second round.

The reigning junior Wimbledon champion, who trains in Montreal, started brilliantly when claiming the first nine points of the match before Peer steadied.

But Bouchard, a powerful ball striker off both wings, continued to play aggressively and showed poise deep in the third set to set up a meeting with China's Li Na.